I think we have all seen those "invincible" units. The ones that can out-shoot, out-melee, or simply out-last anything else on the table. The are frequently a powerful beat stick that your opponent can use to hammer your army into dust, where your lowly troops barely have a chance! I had been on the receiving end of those "Deathstar" units on many occasions, and I did not like the chunky, meat paste it made of my troops. I quickly started to think that if a lowly conscript did not have at least the ghost of a chance at defeating it with a crazy luck or massively out-numbering than it was "unfun".
If you read many of my rules, you will find the ability to make "Invincible" Units that can not be killed. You could use combinations of traits, stat lines and command points to buff units to invincible status, especially against other non-"invincible" units.
Now, let's fast forward to my most recently published book, Wars of the Republic. Roman Legionary troops can go into a formation called Legion. This formation gives them powerful armor bonuses in exchange for some reduced mobility. However, the concern is that the Armor rating for this formation can make them essentially immune to other unit's attacks! This frequently came up in play testing, and was a concern for some of the play testers.
Oh no! It looks like I have fallen into the trap and designed "Invincible" units!